Ericsson, whose team-mate Jolyon Palmer won last time out at Monaco, lapped in 1m48.088s, outpacing series leader Davide Valsecchi of DAMS by 0.163s.
Giedo van der Garde was third fastest for Caterham, despite a scary moment when a weaving Fabrizio Crestani didn't see him coming and forced him into evasive action.
Palmer was fourth fastest, to underline iSport's recent renaissance, just 0.002s off van der Garde's time. Monaco race 1 winner Johnny Cecotto Jr was fifth quickest for Addax, ahead of Carlin's Max Chilton.
Fabio Leimer was next up for Racing Engineering, ahead of James Calado (Lotus GP), Felipe Nasr (DAMS) and Stefano Coletti (Scuderia Coloni).
It was a poor session for Luiz Razia, who is second in the championship. He was only 13th fastest in his Arden International machine.
The session was red-flagged halfway through, when Caterham's Rodolfo Gonzalez spun into the barriers as he put the power down exiting Turn 13. The delay while his car was removed cost his peers six minutes of track time.